1. Technical Field
This disclosure relates generally to current detector circuit technology, and more particularly to a current detector circuit having excellent temperature characteristics and a current-mode DC-DC converter using the same.
2. Description of the Related Art
FIG. 1 is a diagram showing a conventional current detector circuit that detects a current flowing through a switching device M11 at the output stage of a step-down DC-DC converter.
In this conventional case, a series circuit of a resistor Rs and a PMOS transistor M12 is connected in parallel to the switching device M11 using a PMOS transistor. Further, the gates of the switching device M11 and the PMOS transistor M12 are connected in common.
The switching device M11 and the PMOS transistor M12 are connected to ground through an NMOS transistor M16. One end of the NMOS transistor M16 is grounded, and the output voltage Vout of the DC-DC converter through a smoothing circuit formed of an inductor L1 and a capacitor C1 is extracted from the other end of the NMOS transistor M16.
A signal P applied to the gates of the switching device M11 and the PMOS transistor M12 and a signal N applied to the gate of the NMOS transistor M16 are complementary to each other. A current detection signal Vsense is extracted from the connection node of the resistor Rs and the PMOS transistor M12. The above-described complementary signals P and N are generated based on this current detection signal Vsense.
The current detection signal Vsense in this circuit is expressed as follows:Vsense=IL·Rm1on·Rs/(Rm1on+Rm2on+Rs),  (1)where IL is a load current, Rm1on is the ON-resistance of the switching device M11, and Rm2on is the ON-resistance of the PMOS transistor M12.
Normally, MOS transistors have temperature coefficients greater than that of the resistor Rs formed of a diffused resistor or the like. Therefore, Eq. (1) is rewritten into an equation in consideration of temperature, which is expressed as follows:Vsense=IL·Rm1on(1+Δt·γ)·Rs/(Rm1on(1+Δt·γ)+Rm2on(1+Δt·γ)+Rs),  (2)where Δt is a change in temperature, γ is the temperature coefficient of the ON-resistance of each MOS transistor, and the temperature coefficient of the resistor Rs is 0.
Dividing each of the numerator and the denominator of Eq. (2) by (1+Δt·γ) results in:Vsense=IL·Rm1on·Rs/(Rm1on+Rm2on+Rs/(1+Δt·γ)),  (3)thus leaving the term (1+Δt·γ) showing temperature dependence in the equation. This shows that the current detection signal Vsense has temperature dependence. Therefore, temperature compensation is necessary in current detector circuits having circuit configurations as described above.
FIG. 2 is a diagram showing a current detector circuit disclosed in Japanese Laid-Open Patent Application No. 2000-307402.
As shown in FIG. 2, in this current detector circuit, a series circuit of an NMOS transistor M22, the resistor Rs, and a heat-sensitive element 5 is connected in parallel to an NMOS transistor M21, so that temperature compensation of the NMOS transistor M21 is performed with the heat-sensitive element 5.
The current detection signal Vsense is output from the connection node of the resistor Rs and the heat-sensitive element 5. A thermistor, diode, or MOS transistor is employed as the heat-sensitive element 5.
FIG. 3 is a diagram showing an overcurrent protection circuit disclosed in Japanese Laid-Open Patent Application No. 2002-26707.
As shown in FIG. 3, in this overcurrent protection circuit, a series circuit of an NMOS transistor M32 and the resistor Rs is connected in parallel to an NMOS transistor M31, so that the current flowing through a load element 20 branches into a current Id1 and a current Id2. The gates of the NMOS transistors M31 and M32 are connected to a gate drive circuit 50.
In FIG. 3, a series circuit of diodes 32 is a first constant voltage power supply having temperature dependence, and Vref1 denotes a second constant voltage power supply having very little temperature dependence. The voltage difference between the first and second constant voltage power supplies is amplified in an operational amplifier circuit 31 to generate a temperature-dependent reference voltage, and the generated reference voltage is applied to one input of a comparator 34.
The current detection signal Vsense, which is the voltage drop across the resistor Rs, is applied to the other input of the comparator 34. The NMOS transistors M31 and M32 and the series circuit of multiple diodes 32 are integrated onto a same semiconductor substrate 30. The output of the comparator 34 is input to the gate drive circuit 50.
According to the above-described configuration, this overcurrent protection circuit compensates for the temperature characteristics of the ON-resistances of the NMOS transistors M31 and M32 with the temperature characteristics of the first constant voltage power supply 32 formed of diodes.
In the case of detecting a detection current that changes with temperature by converting the detection current into voltage with a resistor having little temperature dependence, the detection signal has temperature dependence. Therefore, in the above-described conventional techniques, a component having opposite temperature characteristics (a heat-sensitive element such as a diode, thermistor, or MOS transistor) is added in order to compensate for the temperature dependence.
However, according to the above-described conventional techniques, a device used in the current detector circuit (an NMOS transistor in the above-described conventional techniques) and an element used for temperature compensation (a heat-sensitive element such as a diode, thermistor, or MOS transistor) are different, thus causing a problem in that complete temperature compensation cannot be performed.